View
238
Download
0
Category
Preview:
Citation preview
Power System Protection Part – VIII Dr.Prof.Mohammed Tawfeeq Al-Zuhairi
133
Transmission Lines and Feeders Protection
Pilot wire differential relays (Device
87L)
Distance protection
Power System Protection Part – VIII Dr.Prof.Mohammed Tawfeeq Al-Zuhairi
134
1. Pilot wire differential relays (Device 87L)
The pilot wire differential relay is a high-speed relay designed for protection of
transmission and distribution lines. They are generally applied on short lines,
normally less than 40 km long.
The scheme requires communication channel (link) to carry system voltage and
current information to the control location. The main objective of using pilot
relaying is to remote control of the circuit breakers.
Four basic communication channels are used:
1. Separate telephone circuit (telephone wire or cable) this is called pilot wire
carrier.
2. Microwave system using directional dishes.
3. Fibre optic cable.
4. Power line carrier.
(a) Operating principles of a current pilot wire relay
Pilot wire differential relaying is a relay system consisting of two identical
relays located at each end of a line (see Figure 1). The relays are connected
together with a two-conductor pilot wire. The output from three individual
phase CTs is applied to a summing transformer that produces a composite
current which is proportional to the line current and has a polarity related to line
current flow direction.
The circuit is basically that of the percentage (restraint) differential relay with
the operating circuit broken into parallel circuits separated by pilot wires. This
relay is available in both electromechanical and static designs.
When the fault is external to the relay’s protective zone, current flows
in the pilot wire through each relay’s restraint coils, but not through the
relay’s operating coil.
If the fault is within the relay’s protective zone and current is flowing
into the fault from both directions, the direction of pilot wire current IPA
remains the same; but the direction of current IPB reverses and forces
current to flow into each relay’s operating coil. If the fault current flows
through circuit breaker A only, the relay at A still passes sufficient
current through the pilot wire to operate the relay at circuit breaker B.
Power System Protection Part – VIII Dr.Prof.Mohammed Tawfeeq Al-Zuhairi
135
Summing transformer is used to convert current signal to voltage signal.
For faults outside the line from both sides at F1 and F3 , VA = VB , relay
will not operate.
For fault inside the line at F2 , relay will operate, since VA ≠ VB.
The pilot wire signal is about 30V ac at 50 Hz or 20V ac at 60 Hz.
Fig.1 Pilot relaying of short transmission line . R=voltage restrain coil; O=
current operating coil.
(b) Power Line Carrier (PLC)
In power line carrier protection scheme, a high frequency signal in the band of
80-500 kHz and of low power lever is transmitted via the power line conductors
from each end of the transmission to the other.
Signal is received by either end of the line by a receiver R to give tripping or
blocking orders to the circuit breaker. The system is shown in Fig.2.
The high frequency is injected to the power line by a coupling capacitor.
The signals are confined to the line by an LC blocking filter at each end.
This is called a line trap. Line trap is shown in Fig.3.
This means that the power transmission line is used as the communication
channel (circuit).
Power System Protection Part – VIII Dr.Prof.Mohammed Tawfeeq Al-Zuhairi
136
Fig.2 Power line carrier system.
Fig.3 A 500kV Line Trap.
Power System Protection Part – VIII Dr.Prof.Mohammed Tawfeeq Al-Zuhairi
137
2.Transmission Line Protection: Distance Relay
Transmission line protection by pilot wires (pilot relaying) is limited to 30 to 40
km in rout length. For longer transmission lines and subtransmission lines or
even distribution feeders, distance protection is used.
Principle of Distance Protection
The term distance is used for a family of relays that respond to a ratio of
voltage to current and therefore to impedance or component of
impedance .Distance relay may have the following features:
A distance relay has the ability to detect a fault within a pre-set distance
along a transmission line from its location.
Every power line has a resistance and reactive per kilometer related to its
design and construction so its total impedance will be a function of its
length or distance.
A distance relay therefore looks at current and voltage and compares
these two quantities on the basis of Ohm’s law (see Figure 1).
Consider the simple radial line with distance protection system installed at the
end A (the local end) while end B is called the remote end. These relays sense
local voltage and current and calculate the effective impedance at that point.
This means that the relay requires voltage and current information.
Fig.1
Power System Protection Part – VIII Dr.Prof.Mohammed Tawfeeq Al-Zuhairi
138
When the protected line becomes faulted, the effective impedance becomes the
impedance from that point to the fault.
Assume balanced three-phase fault at distance d:
For internal fault at point p :
Relay will operate
For external fault at point k :
Relay will not operate
In general the relay will trip when
where Zf is the impedance at
the fault point which is line length. For example at point p:
.
Hence the distance relay action is to compare the local voltage with the local
current, i.e. the secondary values of V and I in the voltage and current
transformers so as the quantity
…is the measured impedance Zm where,
Where Zm is the measured impedance (appears at the relay terminal) also called
the secondary impedance.
and Zf is the also called the primary impedance.
Types of Distance Relays
In general there are four main types of distance relays
Impedance relay (Ohms relay)
Admittance relay ( Mho relay)
Reactance relay
Offset Mho relay
Impedance relay can also be of two types: a plain relay or directional
type. It can be constructed as electromagnetic like the balanced beam
relay or static type like the bridge comparator relay. Also digital
impedance relay is available.
Admittance relay can be either static or digital types.
Power System Protection Part – VIII Dr.Prof.Mohammed Tawfeeq Al-Zuhairi
139
Plain impedance relays:
1 - Balanced beam relay
The concept can best be appreciated by looking at the pioneer-type balanced beam relay (see Figure 2). The voltage is fed onto one coil to provide restraining torque Tr , whilst the current is fed to the other coil to provide the operating torque To.
Under healthy conditions, the voltage will be high (i.e. at full-rated level), whilst the current will be low (at normal load value), thereby balancing the beam, and restraining it so that the contacts remain open.
Under fault conditions, the voltage collapses and the current increase dramatically, causing the beam to unbalance and close the contacts.
Voltage coil Current coil
Fig.2 Balanced beam relay used as a distance relay.
For voltage coil : Tr = K1 V2
For current coil : To = K2 I2
Where K1 and K2 are constants .
For balance case: Tr = To or K1 V2 = K2 I
2
For the contact of relay to close: Tr ˂ To or K1 V2 ˂ K2 I
2
or
Hence, the relay will operate when the impedance it ‘sees’ is less
than a predetermined value.
Power System Protection Part – VIII Dr.Prof.Mohammed Tawfeeq Al-Zuhairi
140
Tripping characteristics of distance relay
If the relay’s operating boundary is plotted, on an R-X diagram, its
impedance characteristic is a circle with its center at the origin of the
coordinates and its radius will be the setting in ohms (Figure 3).
The relay will operate for all values less than its setting i.e. is for all
points within the circle. This is known as a plain impedance relay and it
will be noted that it is non-directional, in that it can operate for faults
behind the relaying point. It takes no account of the phase angle
between voltage and current.
Fig.3 Plain impedance characteristic.
2. Bridge comparator static impedance type distance relay
A more modern technique for achieving the same result is to use a
bridge comparator distance relay (see Figure 4).
Referring to Fig.4:
VT output is converted to a current IR which is the restrain current due to ZR .
CT provide the operating current Io .
IR and Io are converted to scalar values by two rectifiers , and the relay current is the numerical difference of the two currents .
Power System Protection Part – VIII Dr.Prof.Mohammed Tawfeeq Al-Zuhairi
141
If the current through the relay is greater than the setting current , it will cause the relay to close the contacts (operates).
If at the remote point B, the restrain current is approximately equal to the operating current, the relay will not operate.
During an internal fault (0 – 80% in Zone 1) , the relay current should be several times the setting current to ensure its fast operation, the operation should be within 60 ms time .
Fig.4 Bridge comparator in modern distance relay.
Notes :
The impedance relay does not consider the phase angle between the voltage and current applied to it.
The relay operates for all impedance values that are less than its setting (all the values inside the circle shown in Fig.3).
It restrain for all values (points) outside the circle.
Power System Protection Part – VIII Dr.Prof.Mohammed Tawfeeq Al-Zuhairi
142
Relay Reach, underreach and overreach
The reach of the distance relay is that distance from the relaying point to
the point of fault. The reach is usually refers as the relay setting and can
be as a distance (m), or as a primary or secondary impedance.
Relay reach adjustment
Referred to Fig.2, by changing the ampere-turns relationship of the
current coil to the voltage coil, the ohmic reach of the balanced - beam
relay can be adjusted. A more modern technique for achieving the same
result is to use a bridge comparator (see Figure 4).
Advantage of distance relay:
1. Provide backup protection easily.
2. Eliminates the pilot channel.
Features
Distance protection is available for both phase and ground faults. Step distance protection combines instantaneous and time delay
tripping.
Zones of protection
Due to the tolerance in the circuit components, the measuring
accuracy cannot be perfect so it is usual to set the relay at the
local point A at 80% of the secondary impedance of AB. This is
referred as zone 1 or stage A1 setting (see figure 5).
The remaining 20% of AB is protected by by changing the setting
of the relay to reach 50% into zone BC (zone 2 or stage A2). Stage
A2 is usually set at 0.3 s time.
For system reliability (failure of relay A will cause failure of stage
A1 and stageA2), another distance relay is added for backup
protection. This separate relay should have a reach of 20% into
CD and called zone 3 or stage A3 which has a time delay of 0.6 s.
Power System Protection Part – VIII Dr.Prof.Mohammed Tawfeeq Al-Zuhairi
143
Fig.5 A three- sage distance protection system.
Notes:
Zone 1 is an underreaching element, any fault within Zone 1 is known to be on the protected line. When Zone 1 operates, the line is tripped instantaneously.
Zone 2, however, will operate for some external faults.
Summary:
Discrimination zone (or setting zone) by:
%)250%200(
%)150%120(
%)85.0%80.0(
3
2
1
protectionZ
protectionZ
protectionZ
Power System Protection Part – VIII Dr.Prof.Mohammed Tawfeeq Al-Zuhairi
144
Example 1: For the 66kV radial feeder shown in Fig.6, Calculate zone
1 setting for the distance relay in primary ohms.
Fig.6
Example 2:
Figure 7 shows a simple two radial lines. We will consider the settings
for line AB at bus B. The impedance angle for each line is 75°. The line
length is 80 km, the distance relay at bus A is fed by current
transformers rated at 2000 A: 5 A and voltage transformers rated at 345
kV/200 kV Y: 120 V/69 V Y. Find the settings of zone 1 and zone 2 of
the relays.
Solution
Set Zone 1 for 85 %:
Zone 1 setting = 0.85 x 80 = 68 ohm, primary ohm setting (Zfp)
CT ratio = 2000/5 = 400
VT ratio = 200,000/69 = 2900
Relay setting for zone 1 = Zfp . CT ratio/VT ratio
= 68 (400)/(2900)
= 9.38 relay ohms
Power System Protection Part – VIII Dr.Prof.Mohammed Tawfeeq Al-Zuhairi
145
Fig.7
Zone 2 setting : 120 % -150% Choose 140% ;
Zone 2 setting _=1.40 x 80 = 112 ohm, primary ohm setting (Zfp)
Relay setting for zone 2 = Zfp . CT ratio/VT ratio
=112 (400)/(2900)
= 15.44 relay ohms
Example-3 Consider the 230-kV transmission system shown in Fig.8.
Assume that the positive-sequence impedances of the lines L1 and L2
are 2 + j 20 Ω and 2.5 + j 25 Ω, respectively. If the maximum peak load
supplied by the line L1 is 100 MVA with a lagging power factor of 0.9,
design a three-zone distance-relaying system for the R12 impedance
relay by determining the following:
(a) Maximum load current
b) CT ratio and VT ratio )
(c) Impedances measured by relay
d) Zone 1 setting of relay R12 )
e) Zone 2 setting of relay R12)
(f) Zone 3 setting of relay R12
(g) Draw the zones of protection for R12 and suggest time settings for
the three zones.
Power System Protection Part – VIII Dr.Prof.Mohammed Tawfeeq Al-Zuhairi
146
Fig.8
Solution :
(a) Max.load current =
= 251.02 A
(b) Choose: CT ratio = 250 / 5
VT ratio =
(c) Impedances measured by relay is
Impedance of line L1and line L2 as seen by the relay (Line impedances
based on secondary ohms) are:
Zsec - L1 = 0.026 (2+j20) = 0.52 + j 0.5196 Ω
Zsec – L2 = 0.026 (2.5+j25) = 0.65 + j 0.6495 Ω
(d) Zone 1 setting of relay R 12 is
Z1 = 0.8 (0.52 + j 0.5196 )= 0.0416 + j 0.4157 sec. Ω
(e) Zone 2 setting of relay R 12 is
Z2 setting is = 120% - 150% , Choose 140%
Z2 = 1.4 (0.52 + j 0.5196 )= 0.0728 + j 0.727 sec. Ω
(f) Zone 3 setting of relay R12 : Since the zone 3 setting must reach
beyond the longest line connected to bus 2 ,thus
Z3 = 120% (Zsec - L1 + Zsec – L2) = 1.20 (0.52 + j 0.5196 + 0.65 + j
0.6495) = 0.1400 +j 1.402 sec. Ω
Power System Protection Part – VIII Dr.Prof.Mohammed Tawfeeq Al-Zuhairi
147
(g) Draw the zones of protection for R12 and suggest time settings for
the three zones.
Fig.9
Is the distance relay of plain impedance type is directional?
The impedance relay is non-directional that is, it operates for all faults along the
vector AB in Fig.10 such as point p and also for all faults behind the bus A up
to impedance AC such as point k.
Fig.10
A directional unit may be added to the plain impedance distance relay to make it
operate in one direction only as depicted in Fig.11.
Power System Protection Part – VIII Dr.Prof.Mohammed Tawfeeq Al-Zuhairi
148
Fig.11 Typical per-phase arrangement for a three – zone distance relay with
directional unit .The directional unit may be a wattmetric relay.
Recommended